The most common confusion about Alsup's decision, in my view, comes from
the imprecision of programmers' use of the term “API”. The API
and the implementation of that API are different. Frankly, in the free
software community, everyone always assumed APIs themselves weren't
copyrightable. The whole idea of a clean-room implementation of something
centers around the idea that the APIs aren't copyrighted. GNU itself
depends on the fact that Unix's APIs weren't copyrighted; just the code
that AT&T wrote to implement Unix was.

Those who oppose copyleft keep saying this decision eviscerates copyleft.
I don't really see how it does. For all this time, free software advocates
have always reimplemented proprietary APIs from scratch. Even copylefted
projects like Wine depend on this,
after all.

But, be careful here. Many developers use the phrase API to mean
different things. Implementations of an API are still copyrightable,
just like they always have been. Distribution of other people's code
that implement APIs still requires their permission. What isn't
copyrightable is general concepts like “to make things work, you need
a function that returns an int and takes a string as an argument and
that function must called Foo”.